The New Austrian Tunnelling method (NATM) was developed between 1957 and 1965 in Austria. It was given its name in Salzburg in 1962 to distinguish it from old Austrian tunnelling approach. The main contributors to the development of NATM were Ladislaus von Rabcewicz, Leopold Müller and Franz Pacher. The main idea is to use the geological stress of the surrounding rock mass to stabilize the tunnel itself.[1]
Many have argued that the New Austrian Tunnelling method was not new or Austrian having been previously used else where in Europe and isn't a tunnelling method as much as a philosophy. This aside NATM has no doubt done much to revolutionise tunnelling and bring it into the 21st century
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The NATM integrates the principles of the behaviour of rock masses under load and monitoring the performance of underground construction during construction. The NATM is not a set of specific excavation and support techniques and has often been referred to as a "design as you go" approach to tunnelling providing an optimized support based on observed ground conditions but more correctly it is a "design as you monitor" approach based on observed convergence and divergence in the lining as well as mapping of prevailing rock conditions.
There are seven features on which NATM is based:
Based on the computation of the optimal cross section, just a thin shotcrete protection is necessary. It is applied immediately behind the Tunnel boring machine, to create a natural load-bearing ring and therefore to minimize the rock's deformation. Additionally, geotechnical instruments are installed to measure the later deformation of excavation. Therefore a monitoring of the stress distribution within the rock is possible.
This monitoring makes the method very flexible, even at surprising changes of the geomechanical rock consistency during the tunneling work, e.g. by crevices or pit water. Such (usual) problems are not solved by thicker shotcrete, but the reinforcement is done by wired concrete which can be combined with steel ribs or lug bolts.
The measured rock properties lead to the appropriate tools for tunnel strengthening. Therefore in the last decade NATM was also applied to soft ground excavations and to tunnels in porous sediments. The flexible NATM technique enables immediate adjustments in the construction details, but this requires a flexible contractual system, too.
NATM was originally developed for use in the Alps where tunnels are commonly excavated at depth and in high in situ stress conditions. The principles of NATM are fundamental to modern day tunnelling, however most city tunnels are built at shallow depth and need not control the release of the in situ stress, seeking instead to minimise settlement. This has led to a confusion in terminology in that tunnelling engineers use "NATM" to mean different things: some define it as a special technique, but others as a sort of philosophy. Recently the scene has been complicated by new terms and alternative names for certain aspects of NATM. This is partly caused by an increased use of the method in the USA, particularly in soft ground shallow tunnels (see External links).
Besides the official name New Austrian Tunnelling Method other designations are used, e.g. Sequential Excavation Method (SEM) or Sprayed Concrete Lining (SCL) are often used in shallower tunnels. In Japan sometimes other names were used, e.g. Centre Dividing Wall NATM, or Cross Diaphragm Method (both abbreviated as CDM), and even Upper Half Vertical Subdivision method (UHVS).
The Austrian Society of Engineers and Architects defines "NATM" as a method where the surrounding rock or soil formations of a tunnel are integrated into an overall ring-like support structure. Thus the supporting formations will themselves be part of this supporting structure.
However, many engineers use "NATM" whenever shotcrete is proposed for initial ground support of an open-face tunnel. Especially with reference to soft ground, the term NATM can be misleading. As noted by Emit Brown, NATM can refer to both a design philosophy and a construction method.
According to E.Brown (Weblink 2), the key features of the design philosophy refer to:
When NATM is seen as a construction method, the key features are:
Some experts note that many of these construction methods were used in the US and elsewhere in soft-ground applications, before NATM was described in the literature.
In an article of 2002 Romero states the major difference between the viewpoints of design and of construction: The deformation of the soil (rem.: at soft-ground tunnels) is not easily ‘controlled’. Therefore it can be concluded that the excavation and support planned for sequentially excavated, shotcrete-lined tunnels .. utilizes NATM construction methods but not necessarily NATM design methods. These details are less essential at tunnels in solid or fair rock.